1. Field of the Invention
This invention relates to an apparatus by which a plurality of high speed external computer data storage devices (e.g. computer hard drives) that are housed within portable carriers can be effectively coupled to and operated from the same low voltage differential (LVD) SCSI bus of a host computer by means of electrically connecting a repeater chip to each of the data storage devices.
2. Background Art
Computer systems are known having a plurality of external data storage devices (e.g. computer hard drives). For low voltage operation, it is desirable that all of the data storage devices be connected directly to the low voltage differential (LVD) SCSI bus of a host computer. However, when the external data storage devices are housed within removable carriers (i.e. portable carriers that are adapted to be removed from a multi-bay chassis), problems have arisen when many of such high speed data storage devices are connected to and operated from the same bus. More particularly, system delay and signal error have been known to occur when more than two or three removable data storage devices are operating at the same time and at full speed (e.g. 80 MHz per second). Such problems are a consequence of signal distortion, impedance mismatches and parasitic capacitance that are inherently produced by the relatively long stub length that is commonly required for coupling a removable data storage device to its LVD SCSI bus.
Accordingly, it would be desirable to be able to overcome the aforementioned problems by making the stub length between an external data storage device and its LVD SCSI bus appear as short as possible to the SCSI controller so that many high speed data storage devices that are housed within respective removable carriers at different chassis can be efficiently coupled directly to the same bus and operated at full speed at the same time.
In general terms, apparatus is disclosed by which a maximum number (e.g. 16) of external computer data storage devices that are housed in respective removable carriers can be effectively coupled to and operated from the same low voltage differential (LVD) SCSI bus by eliminating the signal distortion that is introduced as a consequence of the relatively long stub lengths that are commonly required for connecting the external data storage devices in removable carriers to the bus. A chassis has a series of drive bays in which are stationed U-shaped receiving frames stacked one above the other. Each U-shaped frame accommodates a removable carrier within which an external computer data storage device (e.g. a hard drive, tape drive or the like) is housed. The data storage device is connected to the backplane at the rear of the U-shaped frame by way of the usual arrangement of connectors and cables that are commonly referred to as a stub length.
According to the present improvement, an isolator board is mated to the backplane at the rear of each U-shaped frame in the multi-bay chassis. The isolator board carries a repeater chip. With the removable carrier returned to its U-shaped receiving frame at a drive bay of the chassis, the repeater chip is electrically connected between the LVD SCSI bus and the external computer data storage device that is housed within the removable carrier, whereby to modularize a network having many data storage devices connected to the same LVD SCSI bus. The addition of a repeater chip connected to a data storage device avoids the adverse effects commonly caused by the stub length for coupling the storage device to its bus by making it appear to the SCSI controller that the stub length is infinitesimally short and that the data storage device is connected directly to the bus. The repeater chip causes all of the data and control signals transmitted to the data storage device over the LVD SCSI bus to be repeated (i.e. started over at the isolator board with their original signal format and quality) as to avoid signal distortion or loss and make it is possible to efficiently operate as many as sixteen different data storage devices at full speed from the same LVD SCSI bus.